1. Field of the Invention
The present invention relates to an optical encoder.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2005-207822 discloses an optical encoder in which a light is illuminated from a light source onto a movable transmission slit to enter a light-receiving element array while being periodically cut off by the transmission slit. Although a light-and-dark distribution is generated at a pitch of slit on the light-receiving element array, it has a complicated waveform that contains a harmonic component such as a triangular wave shape or a trapezoidal shape by the influence of a diffraction of the slit.
When the light-and-dark distribution light having the complicated waveform containing the harmonic component is detected by four-phase light-receiving elements to be converted into A-phase and B-phase differential signals to observe a Lissajous waveform using an oscilloscope or the like, a waveform that has not a circular shape is obtained. Particularly, due to an amount and a phase of a 3rd order harmonic component, the Lissajous waveform becomes the trapezoidal shape or the triangular shape. Periodic signals of the encoder, treated as two-phase sine wave and cosine wave signals (A-phase and B-phase signals), are used for calculating arctangent of the A-phase and B-phase signals. However, if the Lissajous waveform is not the circular shape (there are a lot of distortions in the A-phase and B-phase signals), a measurement error occurs.
Japanese Patent Laid-Open No. H03-128418 discloses a configuration of correcting a shape of a light-emitting region using a mask to bring the waveform of the light-and-dark distribution illuminated on a light-receiving element array closer to a sine wave. However, the state of the light-and-dark distribution varies in accordance with parameters such as a spread of a wavelength, a method of illuminating a light from a light source, a state of an edge of a slit, a distance between the slit and the light-receiving element, and the like, as well as the size or the shape of the light-emitting region of the light source. Therefore, particularly, in an environment where a state of the distance between the slit and the light-receiving element varies, a stable correction was difficult.
Thus, since errors of amplitude, an offset, a phase, and a harmonic distortion are contained in an encoder signal or an interference measurement apparatus, a signal correction is necessary in order to perform a high-precision measurement. Particularly, in a case of a slit transmission encoder or an interference fringe projection encoder, a harmonic distortion component significantly varies due to a gap variation between the scale and the light-receiving element. Therefore, the high-precision measurement by interpolation was difficult. The technology that removes the harmonic distortion contained in a sine wave signal has also been requested in a field of the Michelson interfering measurement apparatus.